Watermarking is a technique whereby a payload is represented as one or more watermark 20 patterns that are subsequently embedded in content such as movies, pictures, songs, or radio and television shows. The content is then distributed e.g. by broadcast, streaming, downloading or similar technique or on a storage medium such as a CD, DVD, Blu-Ray disc, flash memory or hard disk. The embedding is usually imperceptible: a person who views or hears the content generally will be unable to pick up the embedded pattern. In some cases, 25 some small parts of the embedded patterns may be perceptible, e.g. as minor degradations in audio or video quality. This pattern is typically a (pseudo)random pattern, although this may depend on the application and technique for embedding and detecting. However, specialized detection tools can reconstruct the pattern and recover the payload from the reconstruction. Watermarking has many applications, for example tracing the source and 30 distribution path of content items, audience measurement, synchronization of media streams, enriching content with metadata or providing a basis for digital rights management.
A watermarking application consists of two stages. The first is an embedder, which adds a watermark to the content. During this embedding stage, a certain message or an amount of 35 information (often referred to as ‘payload’) is represented as a prescribed set of changes that are applied to the content. The second stage is a detector, which extracts the information that was embedded in the content.
One important class of watermarking techniques is referred to as spread-spectrum watermarking. A characteristic property of spread-spectrum watermarking methods is that a relatively small amount of information (the payload) is represented by a sequence of patterns which often have a large bandwidth (e.g., the embedded pattern comprises non-zero components across a wide range of signal frequencies), such that the associated watermark energy density can be very small. In an additive spread-spectrum audio watermarking application, a payload is represented by two or more pseudo-random patterns that are added (by summation) to the audio (host) signal.
Known additive watermarking algorithms embed patterns that are detected using a matched filter, comprising correlation with a set of candidate patterns. The advantages of such methods are their virtually unlimited set of independent patterns that can be embedded, and the flexibility in terms of pattern properties (size, bandwidth, etc). A disadvantage is that detection is in most cases associated with a significant computational complexity due to the correlation operation. Moreover, as this correlation operation is performed most efficiently in a block-wise manner using a discrete Fourier transform, synchronization of analysis blocks and the watermarks embedded in the audio content is crucial for good detection performance.
An alternative to additive watermarking is phase modulation watermarking. Instead of embedding a pattern by addition of the pattern to the content segment, phase modulation modifies the phase of individual frequency components of the content segment according to the phase of the frequency components of the pattern. This method is described in more detail in M. Arnold, P. Baum, and W. Voessing. “A phase modulation audio watermarking technique”. In S. Katzenbeisser and A.-R. Sadeghi, editors, Information Hiding, volume 5806 of Lecture Notes in Computer Science, pages 102-116. Springer Berlin/Heidelberg, 2009. Just like in the additive embedding method, the detection process of this phase modulation method constitutes correlation with a set of candidate patterns.